[Peter Finke]

We always come across the borders of the structure of an aquarium. It’s completely different than the natural biotopes we try to rebuild in part. Helene makes a good suggestion: Eichen-Extrakt. This is a name (oak-extract) of several German products of different unclear composition (?) which I use sucessfully since years. The main ingredient is phosphoric acid maybe together with humic substances, additionally. While plant-aquarists mock about it (I could understand them) it’s quite a different thing with fish-aquarists. If your aim is to lower pH for longer periods with little expense, it’s a very good method. But beware, some drops only suffice. You must control the pH very closely.
Rising pH can be caused by several causes. The main cause in a normal planted aquarium is the activity of plants which is hard to control. In a blackwater aquarium they must play a minimum role. Therefore floating Ceratopteris is of major value with leaves mostly on the water surface and nevertheless very active roots for taking the excess nitrogen ou of the water. Another cause is gravel or wood. It can hardly be normed. The best is fresh wood from peat regions that has never dried out. Gravel should be used in minimum heights only (in my tanks a few millimeters as a settlement base for nitrification bacteria), for normal planting is out in a blackwater tank.
So, my advice is to take phosphoric acid or a brand of “Eichen-Extrakt” in very small doses. Peat is difficult. The reason is nature on the one hand and the trade on the other. Peat regions are all different. And the trade is not transparent; what sort of peat they use and what they are mixing it with is entirely unclear. Therefore there is very good peat and very bad. Even some natural peat contains substances that lead to pH-rise, although it seems to be ridiculous. You must test the peat beforehand if it is conveniant or not. Take the acid und you are on the right side.